JOINTS_1 and WEIGHTS_1 support

[David Stolarsky]

I have a model where attributes on line

GLTFSceneKit/Source/Common/GLTFUnarchiver.swift

Line 1066 in 32a6c9a

 for (attribute, accessorIndex) in attributes.sorted(by: { $0.0 < $1.0 }) { 

contains keys JOINTS_0 and JOINTS_1 and WEIGHTS_0 and WEIGHTS_1, but it seems GLTFSceneKit only supports JOINTS_0 and WEIGHTS_0.

Is this a shortcoming of SceneKit itself? Or of GLTFSceneKit?

[David Stolarsky]

Model: https://drive.google.com/file/d/1X1L9FUuEKEP13HrMFxKmQmH5RVF7W0Jy/view?usp=sharing

[magicien]

Merged #23. Thanks!